The present invention relates to linear voice coil motors. In particular, the present invention relates to such a motor using a construction that enables use of the motor in environments demanding superior performance at a low relative cost.
A linear voice coil motor is an electromagnetic device in which a voice coil assembly supported on one motor part is excited by a current so as to cause interaction with a magnetic flux emanating from another motor part, thereby creating a force between the two motor parts in a direction normal to the current and flux. The two motor parts are configured to permit limited linear displacement (or other movement) therebetween in reaction to the force. By periodically reversing the direction of current flow, reciprocating movement between the parts can be obtained. In practical applications, typically one motor part is held stationary while the other motor part drives or actuates various devices that require rapid and accurate positioning. Exemplary practical applications include a voice cone of a speaker and the transducer arms of a magnetic disk drive.
Key characteristics of linear voice coil motors include linearity, maximum force, driving efficiency, and durability. In respect to linearity, for positional accuracy and smooth and efficient operation, it is desirable that the displacement force between the motor parts be nearly linearly proportional to the instantaneous coil current throughout the length of the stroke (the total travel between the two ends of a voice coil motor). The ratio of the displacement force to coil current is referred to as the “force constant.” A motor exhibiting a displacement force that varies linearly with coil current will have an unvarying force constant over the length of the stroke. Non-uniformities in the magnetic field along the stroke can degrade the linearity performance of the motor.
In one typical but demanding environment, the linear voice coil motor is used to drive the transducer arms of the disc drive of a portable computer. Here it is desirable that the voice motor exhibit high linearity so that the transducer head can be accurately positioned relative to the disc for reading or writing particular data. At the same time, it is desirable that the motor generate a high stroke force for a given drive current to permit rapid access to various data sectors in a manner consistent with ever increasing disc speeds. It is also desirable that the motor be efficient both from an energy and volumetric standpoint so as to achieve high performance with reduced battery consumption and minimal dimensional footprint and weight. For the sake of reliability, it is further desirable that the motor be durable so as to survive heavy-duty cycle use over protracted operational periods.
FIG. 1 shows a prior art linear voice coil motor 20 with anisotropic magnets circumferentially divided into elongated arcuate segments 22. The elongated arcuate segments 22 are positioned between an outer yoke 24a and an inner yoke 24b (also referred to as a “center yoke” or a “center pole”). A voice coil assembly (not shown) includes a copper voice coil (not shown) attached to or supporting a coil housing or carriage (not shown). The voice coil assembly fits between the elongated arcuate magnet segments 22 and the outer surface of the inner yoke 24b. There is a mechanical cylindrical air gap on both sides of the voice coil and the coil housing that allows them to move in relation with the associated surfaces. It should be noted that the inner yoke, the outer yoke, or the voice coil and the coil housing can be the moving component.
Another prior art linear voice coil motor is described in U.S. Pat. No. 4,888.506 to Umehara et al. (the “Umehara reference”), the disclosure of which is incorporated herein by reference. The Umehara linear voice coil motor includes cylindrical outer and inner yokes with a single anisotropic permanent ring magnet (an anisotropic cylindrical ring magnet) affixed to the inner surface of the outer yoke. This anisotropic cylindrical ring magnet forms a cylindrical air gap with the inner yoke. The yokes are of ferromagnetic material and magnetically coupled so that the magnetic field emanating from the inner surface of the ring magnet radially crosses the cylindrical air gap, passes through the yokes, and returns to the magnet's outer surface. The voice coil (e.g. a copper voice coil) is supported by an insulative bobbin of hollow-core form (e.g. coil housing or carriage) that is mounted for slideable linear movement along the inner yoke so that when the coil is energized and interacts with the magnetic field in the cylindrical air gap, the voice coil assembly is driven back-and-forth depending on the current's direction.
Fabrication of anisotropic magnets (either the elongated arcuate segment version shown in FIG. 1 or the anisotropic cylindrical ring magnet discussed in the Umehara reference) begins with a milled powder, but anisotropic magnets are given their final shape by the application of high pressure and temperature while the material is being acted on by a strong magnetic field so that the particle grains align themselves uniformly to provide a strong field which is well-oriented directionally. Where, for example, the linear voice coil motor is used in the disc drive of a portable computer, the relatively stronger magnetic field produced by anisotropic magnets increases the maximum force and efficiency characteristics of the motor so as to facilitate, as described above, faster drive access, increased miniaturization, and decreased battery consumption. Preferred materials for the permanent magnet include various ferrites or, in particularly demanding environments, rare earth cobalt.
The steps noted above for fabricating these anisotropic magnets touch on the highlights of a fabrication process that, in practice, is much more complex. As shown in FIG. 2, an exemplary process necessary to create an anisotropic magnet may include steps such as milling 26a, molding 26b, profile grinding of the inner diameter 26c, profile grinding of the outer diameter 26d, adhering 26e, slicing 26f, dissolution 26g, surface grinding 26h, corner grinding 26i, pickling 26j, and plating 26k. Conventionally, the fabrication process starts with vacuum induction melting of a carefully optimized blend of alloyable ingredients to form an ingot. This ingot is coarsely ground, then further ground to intermediate grain size, and finally jet milled (milling 26a) to an even finer powder of critical grain size. Screening is used to remove undersized and oversized particles. The resulting highly reactive and combustible powder is stored under special conditions, such as in an argon gas atmosphere, while awaiting further processing. Based on chromatography results, a blend of different powders is combined and die pressed (molding 26b) into roughly the final shape desired while being acted on by a pulsed magnetic field to create a roughly shaped form. In the fabrication of anisotropic magnets, it is during the molding step that the particles are aligned. Introduced deformation stresses and density gradients may result in less than perfect grain alignment. Accordingly, after the pressing, the roughly shaped form is demagnetized and sintered in a high vacuum, whereupon further imperfections may arise if any portions of the roughly shaped form are permitted to enter a liquid phase. The rough form is then quenched and aged. At this stage in processing, dimensional measurements of the material are only approximate due to variations in shrinkage during pressing and sintering. For an anisotropic cylindrical ring magnet such as that shown in the Umehara reference, the inner and outer surfaces of the molded form are then roughly ground to the desired dimensions (profile grinding of the inner and outer diameters 26c, 26d). The form is adhered (adhering 26e) to a support and sliced (slicing 26f) into sections with a diamond saw. The adhesive is dissolved away (dissolution 26g) and the surfaces and corners of the annular ring magnet section are further ground (surface grinding 26h and corner grinding 26i) to the required tolerance. Finally the ring magnet is pickled (pickling 26j) and plated (plating 26k) to prevent chipping, crack formation, and oxidation buildup. These multiple steps require capital-intensive fabrication equipment and the retention of highly trained staff to manage and monitor the process.
FIGS. 3A and 3B show magnets suitable for use in a motor such as that shown in FIG. 1. Specifically, FIG. 3A shows elongated arcuate segments 22 of anisotropic material arranged to form an anisotropic ring magnet structure capable of being radially magnetized as indicated. FIG. 3B shows a single anisotropic cylindrical ring magnet 28 of anisotropic material (e.g. such as the device disclosed in the Umehara reference) capable of being radially magnetized as indicated. FIG. 3C shows the anisotropic cylindrical ring magnet 28 shown in FIG. 3B installed but before installation of the voice coil assembly, the figure showing post magnetization magnetic particle orientation. FIG. 3D shows the anisotropic cylindrical ring magnet 28 shown in FIG. 3B installed, but before installation of the voice coil assembly. FIG. 3D shows post magnetization flux, the radial magnetic field present in the cylindrical air gap produced by the radial magnetization of the anisotropic cylindrical ring magnet 28, and/or the general pattern of the magnetic field as it passes through the cylindrical yoke members 24a, 24b. FIG. 3E shows the magnetic force of the magnetic field in the cylindrical air gap, as indicated in FIG. 3D, as a function of linear displacement in an axial direction (toward axis z) in relation to the cylindrical yoke members 24a, 24b. As can be seen from FIG. 3E, the force constant between the cylindrical yoke members 24a, 24b is relatively level.
There are significant disadvantages associated with the use of anisotropic magnets. For example, the anisotropic magnets conventionally used in linear voice coil motors are relatively expensive to fabricate or purchase, particularly in applications where superior magnetic strength and directional uniformity are specified. Also, it is difficult to vary the process to optimize, for example, the dimensions of the magnets for different applications unless, for example, further capital outlays are made to purchase or lease specially dimensioned pressing and sintering equipment.